1. Field of the Invention
The present invention relates generally to a demodulation apparatus and method for a data communication system employing multi-level modulation, and in particular, to an apparatus and method for calculating an input value to a channel decoder in a demodulator for a data communication system employing 64-ary QAM (Quadrature Amplitude Modulation).
2. Description of the Related Art
In general, a data communication system employs multi-level modulation in order to increase spectral efficiency. The multi-level modulation includes various modulation techniques. Herein, reference will be made to 64-ary QAM, one of the multi-level modulation techniques. As known by those skilled in the art, a 64-ary QAM channel encoder modulates a signal coded by binary encoding and transmits the coded signal to a receiver. The receiver then receives the transmitted modulated signal and decodes the modulated signal through soft decision values decoding in a channel decoder. To perform the decoding, a demodulator of the receiver includes a mapping algorithm for generating soft decision values (or soft values), because the received modulated signal is comprised of an in-phase signal component and a quadrature-phase signal component. Therefore, the demodulator of the receiver includes a mapping algorithm for generating soft decision values each corresponding to output bits of the channel encoder from a 2-dimensional received signal.
The mapping algorithm is classified into a simple metric procedure proposed by Nokia, and a dual minimum metric procedure proposed by Motorola. Both algorithms calculate LLR (Log Likelihood Ratio) values for the output bits and use the calculated LLR values as input soft decision values to the channel decoder. The simple metric procedure, which employs a mapping algorithm given by modifying a complex LLR calculation formula into a simple approximate formula, has a simple LLR calculation formula. However, LLR distortion caused by the use of the approximate formula leads to performance degradation. The dual minimum metric procedure, which employs a mapping algorithm of calculating LLR with a more accurate approximate formula and uses the calculated LLR as an input soft decision value of the channel decoder, can make up for performance degradation of the simple metric procedure to some extent. However, compared with the simple metric procedure, this procedure needs increased calculations, thus causing a considerable increase in hardware complexity.